Perhaps contrary to thgoodman's comments, 12 years ago I was all set to buy a 50A extension but balked at the price. Eventually decided just to use the 30A cord I already had (purchased at 1/3 the price) and have used it from time-to-time over the years, but really not that often and don't recall a situation where I really needed 50A where I couldn't reach it with the main cord. Everyone's situation may differ, but I'm so glad I didn't buy the big cord...initial cost and carrying around another (I have more than a few) bulky, rarely needed item.
If you often camp where an extension is required and/or absolutely need 50A power, then probably a good investment.
I've had no voltage or other power problems using the 30A on the end of the 50A (50') with a standard dogbone, other than keeping the draw less than 30A.